1. Field of the Invention
The present invention relates to circuit modules that include a splitter including a transmission filter and a reception filter with different pass bands from each other.
2. Description of the Related Art
In recent years, mobile communication terminals such as cellular phones and mobile information terminals that support communication using a plurality of communication standards such as the global system for mobile communications (GSM) standard and the code division multiple access (CDMA) standard have rapidly become widespread, and in such mobile communication terminals, transmission and reception of signals of different frequency bands is performed using a common antenna. Therefore, there have been growing demands for there to be further increases in the performance of and reductions in the size of front end modules (circuit modules) such as antenna switches including a splitter (duplexer) that separates a transmission signal and a reception signal having different frequencies from each other.
For example, as illustrated in FIG. 7, a splitter 500, which has a reduced size and increased performance as a result of having first and second filter units 503 and 505 having different signal frequency bands arranged close to each other in such a manner that the characteristics of the filter units 503 and 505 are not degraded, is mounted on a mounting substrate to be included in a circuit module, and as a result, the circuit module can be reduced in size and increased in performance. The splitter 500 of the related art illustrated in FIG. 7 includes the first filter unit 503, which includes a surface acoustic wave (SAW) filter device 502, and the second filter unit 505, which includes a SAW filter device 504, on a mounting surface of a base substrate 501 composed of a resin, a ceramic or a polymer material, such as a printed substrate, an LTCC substrate, an alumina-based substrate, a glass substrate or a composite material substrate, and the first and second filter units 503 and 505 are each provided with passive devices such as chip inductors 506 and chip capacitors 507 that serve as peripheral circuit devices that complement the electrical characteristics of the SAW filter devices 502 and 504.
In order to suppress electromagnetic interference between the inductor devices and prevent signal interference between the filter units 503 and 505, the chip inductors 506, which are arranged so as to be adjacent to each other in a boundary portion 508 between the first filter unit 503 and the second filter unit 505, are arranged on the base substrate 501 such that their magnetic flux directions are substantially orthogonal to each other. By adopting this configuration, even though the filter units 503 and 505 are arranged close to each other, signal interference between the first and second filter units 503 and 505 is prevented and changes in the characteristics of the filter units 503 and 505 are reduced.
The splitter 500 of the related art, for which a reduction in size and an increase in performance have been achieved in this way, is mounted on a mounting substrate and the first and second filter units 503 and 505 of the splitter 500 are respectively utilized as a transmission filter and a reception filter and thereby a circuit module such as an antenna switch is formed.
In addition, the splitter 500 has a transmission terminal for allowing input of a transmission signal from the mounting substrate side to the transmission filter, a reception terminal for allowing output of a reception signal from the reception filter to the mounting substrate side, an antenna terminal (common terminal) that is connected to the output side of the transmission filter and the input side of the reception filter, and a ground terminal. The mounting substrate is provided with a transmission electrode and a reception electrode, which are respectively connected to the transmission terminal and the reception terminal of the splitter 500. Then, a transmission signal is output via the transmission terminal to the transmission filter included in the splitter 500 from the transmission electrode of the mounting substrate and the transmission signal input to the transmission filter is subjected to predetermined filter processing and is then output from the antenna terminal of the splitter 500. In addition, a reception signal is input to the reception filter included in the splitter 500 from the antenna terminal and the reception signal input to the reception filter is subjected to predetermined filter processing and is then output to the mounting substrate side via the reception electrode from the reception terminal of the splitter 500.
There is a risk of a transmission signal, which has been output to the transmission terminal of the splitter 500 from the transmission electrode of the mounting substrate in order to input a transmission signal to the transmission filter, leaking toward the reception electrode, which is provided on the mounting substrate and is for allowing input of a reception signal, and interfering with a reception signal input from the antenna terminal and output to the reception electrode of the mounting substrate from the reception terminal via the reception filter. Therefore, in general, in order to prevent a transmission signal output to the transmission terminal of the splitter 500 from the transmission electrode of the mounting substrate from interfering with a reception signal input from the antenna terminal and output to the reception electrode of the mounting substrate from the reception terminal of the splitter 500 via the reception filter, a ground electrode, which is connected to the ground terminal of the splitter 500, is provided between the transmission electrode and the reception electrode on the mounting substrate.
In recent years, size reduction of mobile communication terminals has been rapidly progressing and there is a demand for further size reduction of a circuit module mounted in a mobile communication device and of the splitter 500 mounted in the circuit module. However, if the splitter 500 is further reduced in size, the spaces between the transmission terminal, which is for allowing input to the transmission filter, and the reception terminal, which is for allowing output from the reception filter, of the splitter 500 become extremely small, and therefore, even though the ground electrode, which is connected to the ground terminal of the splitter 500, is provided between the transmission electrode and the reception electrode provided on the mounting substrate on which the splitter 500 is mounted, there is a risk of a transmission signal output to the transmission terminal of the splitter 500 from the transmission electrode of the mounting substrate leaking into the ground electrode and of the transmission signal that has leaked into the ground electrode traveling along an edge portion of the ground electrode toward the reception electrode side and interfering with a reception signal output to the reception electrode of the mounting substrate from the reception terminal of the splitter 500 via the reception filter. This is an obstacle to size reduction of the splitter 500.